The prior art concerning the melting of reduced iron by a reducing gas generated in situ in a furnace and the recovery of the reducing gas includes followings:
1. Cupola
Coke is burned by hot air to generate a hot gas which is then passed upwardly through a coke-filled layer to melt an amount of iron retained therewith. The by-product gas obtained is a low-calorie gas rich in N.sub.2 and CO.sub.2.
2. Process of Korfstahl, West Germany (JP Openlaying No. 55-94408)
Coal and hydrocarbon-type fuel are gasified by oxygen and steam to form a hot gas which is, in turn, passed upwardly through a coal char fluidized bed to melt semi-reduced iron on the top thereof, and the hot gas is recovered.
3. Process of Stiftersen, Sweden (JP Openlaying No. 49-110519)
Oxygen and hydrocarbon-type fuel, together with semi-reduced iron, are blown into a coke-filled layer or carbonaceous reducing agent-filled layer in which the oxygen and the fuel is burned to generate a hot gas, by which iron is melted. Gas reformation is then effected by steam and carbon, using the sensible heat of the hot gas.
These processes of the prior art have the following disadvantages: In the cupola process, the by-product gas obtained is a low-calorie gas rich in N.sub.2 and CO.sub.2, and cannot be used as a reducing or fuel gas.
The second process designed to melt semi-reduced iron resorts to a system in which a coal-char fluidized bed is formed, and an amount of semi-reduced iron on top thereof is heated and melted by the ascending hot gas.
However, the coal char fluidized bed is unstable and poor in the semi-reduced iron retaining power. Hence, it is not expected to bear the semi-reduced iron on the coal-char fluidized bed for a longer period of time. As a result, the iron should be melted in the possible shortest time with a large amount of the hot gas, which means that the thermal efficiency of melting is low.
The third process resorts to a system in which semi-reduced iron, together with oxygen and hydrocarbon fuel, is blown into a carbonaceous reducing agent-filled layer through tuyeres to burn the hydrocarbon, fuel with oxygen thereby obtaining the hot gas, and the iron is melted by the sensible heat of the resulting hot gas. The gas consumed for melting of semi-reduced iron has a temperature higher than the melting point thereof. Therefore, the combustion heat of oxygen and hydrocarbons is not effectively used for melting of sem-reduced iron. As well-known in the art, the production of pig iron by reduction and melting of iron ores is carried out according to two production systems; one wherein iron ores are gas-reduced in the massive state followed by melting, and the other wherein iron ores are heated and melted and thereafter reduced with the aid of a solid reducing agent. The former system is typically the blast furnace process, and the latter is typically the melting/reducing process.
However, the melting/reducing process has the disadvantages that the reduction of molten iron ores with the solid reducing agent involves a considerably endothermic reaction which renders a stable supply of heat into a reaction bath very difficult, and that there is a marked erosion of refractory materials due to molten iron ores. Hence, in the art no process renders the productivity and economy comparable to those of the blast furnace process.
Like the blast furnace process, on the other hand, the production system in which iron ores are gas-reduced and thereafter melted is advantageous in that the gas reduction of iron ores is a certain exothermic reaction which proceeds in a stable manner, and in that the melt has a reduced content of iron oxides thus posing little or no problem in connection with the erosion of refractory materials in comparison with the melting/reducing process. In addition, the blast furnace process exhibits a very high thermal efficiency due to the fact that the gas-reduction and melting of iron ores are carried out in the same vessel, and reduces the consumption of energy if a by-product gas is recovered for another purposes.
As well-known in the art, however, the blast furnace process requires the use of coke of high quality, such as with high strength or low reactivity, so as to ensure good permeability in the furnace and stable descending of the stock therein. The production of these cokes inevitably needs a feed of coking coal of high quality and high energy for coking. The agglomerated iron ores used should also have a high strength and excel in the softening properties at high temperatures.
There is now an increasing demand for the process of the production of pig iron with the productivity and thermal efficiency bearing comparison with those of the blast furnace process as well as with the possibilities of applying raw materials of low quality. Such a process will be of great significance with the future of natural resources in mind and there have been many attempts to investigate a new process.
In the process for the production of gas by combustion of solid fuels such as coke and coal, in general, the higher the reaction temperature, the better the gasification efficiency will be. With the prior art gasifying furnace, however, it is impossible to effect high-temperature combustion since, as the reaction temperature rises, the resulting ashes are converted into a melt which is very difficult to treat. A typical example of the gasifying process as referred to above is the Lurgi process using a static bed furnace operated under pressure. The Lurgi process is characterized in the use of a static bed furnace, and has the advantages that the gasification temperature is as low as 1100.degree. C.; the removal of ashes is relatively easy; the amount of dust generated is by far less than that in the case of a fluidized bed furnace; and etc. However, this system has the following demerits: The yield of methane is low, resulting in that an appreciable burden is imposed on methanization so that no high calorie-gas is obtainable; neither fine coal nor coking coal is used; difficulties are encountered in the up-scaling of the system size; and the like. In this connection, it is noted that the combustion temperature in the Lurgi process is as low as 1100.degree. C., with the gasification efficiency being low as a consequence, and the resulting gas has a CO.sub.2, content of about 30%, thus remarkedly rich in CO.sub.2.